Cleveland State University EngagedScholarship@CSU

Mechanical Engineering Faculty Publications Mechanical Engineering Department

6-15-2006

An Experimental Investigation of Humidity and Temperature Effects on the Mechanical Properties of Persfluorosulfonic Acid Membrane

Y. L. Tang University of Delaware

Anette M. Karlsson Cleveland State University, [email protected]

Michael H. Santare FUnivollowersity this of and Delawar additionale works at: https://engagedscholarship.csuohio.edu/enme_facpub Michael Part of Gilber the Mechanicalt Engineering Commons HowUniversity does of access Delawar toe this work benefit ou?y Let us know! SimonPublisher Cleghorn's Statement GorNOTICE:e Fuel thisCell Tisechnologies the author ’s version of a work that was accepted for publication in Materials Science and Engineering A. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be See next page for additional authors reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Materials Science and Engineering A, 425, 1-2, (06-15-2006); 10.1016/j.msea.2006.03.055

Original Citation Tang, Y., Karlsson, A. M., Santare, M. H., 2006, "An Experimental Investigation of Humidity and Temperature Effects on the Mechanical Properties of Perfluorosulfonic Acid Membrane," Materials Science & Engineering A, 425(1-2) pp. 297-304.

This Article is brought to you for free and open access by the Mechanical Engineering Department at EngagedScholarship@CSU. It has been accepted for inclusion in Mechanical Engineering Faculty Publications by an authorized administrator of EngagedScholarship@CSU. For more information, please contact [email protected]. Authors Y. L. Tang, Anette M. Karlsson, Michael H. Santare, Michael Gilbert, Simon Cleghorn, and William B. Johnson

This article is available at EngagedScholarship@CSU: https://engagedscholarship.csuohio.edu/enme_facpub/130 An experimental investigation of humidity and temperature effects on the mechanical properties of perftuorosulfonic acid membrane

3 Yaliang Tang 3, Anette M. Karlsson 'l.*, Michael H. Santare , Michael Gilbert a, Simon Cleghorn b, William B. Johnson b • DelXlrlmem ofMec/rall;m/ Engineering. U";"l'rsiry oflJefa"Y,re. DE 19716. Unite(/ Sillies bCOrti. Fuel Cd/ Tedmoiogie,<, 201 Air{IQrt ROlld. P.O. B(JX 1481.l. £Ik/()II, MD 21921-1 4813, United SImes

1. Introduction expansion and contract ion as a functi on oftemperature and mois­ ture. The different thermal ex pansion and swelling coeffi cients Proton exchange membrane fu el cells (PEMFCs) are of these various mate rials introduce hygro-thermal stresses as expected to become a prominent technology in a variety of power the system is cycled, potentia ll y leading to deterioration and generation appli cations, including stationary, ponable, and auto­ eventual malfunction of the fuel cell [3- 51 . A rational approach motive applications. The ir potentially high energy density. high to developing an improved lifetime for PEMFC membranes effic iency and clean operations offer allractive advantages over necessitates that the fai lure mechanisms be clearly understood traditional methods of power generation I i ,2 1. and life prediction models be developed so that new desig ns The cent ral function in a PEMFC is performed bYlhe polymer can be introduced. A critical step toward this understanding electrolyte membrane. In use, the polymer electrolyte mem­ is to full y chamcterize the membrane's mechanical behavior brane serves to conduct proto ns from the anode electrode to under in situ operating environments. Th is work focuses on such the cathode electrode, while acting as an electronic insu lato r characteri zation. and a gas barri er to prevent mixing of oxygen and hydrogen Several publications have reported the importance of the (Fig. I). Any discontinuity of the membrane will compromise membmnes mechan ical properties on operational life [3-91. the fun ction of the fu el cell. A typical auto motive duty cycle Me mbrane fa ilures in the membrane electrode assembly (MEA) imposes rapid changes in the c urrent density and voltage of have been precipitated solely by cycling between weI and dry the fu el cell. as well as rapid changes in the cell temperature, operating conditions without electric potential or reacti ve gases pressure and relative humidity. The polymer electrolyte mem­ [31. These fa ilures can appear as pinholes in the membrane brane. gas diffusion layers and graphite plates will all experience or de lamination between polymer membrane and gas diffu sion layers l6,7j. Several analytical studies l4,5,8j have shown how hygro-themlal stresses might plllY an important role in these fail­ • Corresponding author. Tel.: +1 302831 6437: fax: + I 302 831 3619. ures. Thus, it is important to characterize the hydro-mechani cal E' IIwi/ (l(/dress: Karlsso n@rnc .udcl.cdu (A.M. Karlsson). properties of the constituents. rinsed with DI water. The pre-treatment was conducted to obtain a common material state for all samples. After pre-treatment, the membrane was dried at room temperature for 24 h, and then cut along both the machine and transverse directions into rectangu­ lar specimens with length of 100 mm and width of 10 mm.

2.2. Experimental apparatus

Tensile tests were conducted with an MTS AllianceTM RT/5 material testing system with an ESPEC custom-designed envi­ ronmental chamber (Fig. 2A). The chamber can operate in condi­ tions from –70 to 170 ◦C and relative humidity between 30% and 95%. Besides the sensors installed in the chamber to control the temperature (copper/copper–nickel type ‘T’ thermocouple) and humidity (Rotronic H290 humidity transmitter) of the chamber, an additional Testo 645 humidity sensor was positioned close to the specimen in order to measure the temperature and humidity as close to the specimen as possible.

2.3. Testing procedures Fig. 1. Schematic of a proton exchange membrane fuel cell. Membrane properties were measured at 16 temperature and humidity combinations, i.e., at four different temperatures (25, PEMFCs operate at temperatures ranging from ambient to ◦ 100 ◦C and a variety of humidification levels. The membranes 45, 65, 85 C) and four different relative humidities (30%, 50%, absorb up to 50 wt.% water and experience significant volu­ 70%, 90%) each. Five specimens were tested at each tempera­ metric swelling. Upon moisture absorption, the strength of the ture and humidity combination. For each specimen, the thickness membrane drops dramatically due to the plasticization of the and width were taken as the average of three measurements dis­ ionomer [10]. Although effects of temperature and humidity on tributed over the sample. The specimen was aligned with the the strength of PFSA membrane have been reported [11,12], extension rod by a pair of vise-action grips (Fig. 2B), and the to the knowledge of the authors, only conditions at ambient gauge length was adjusted to 50 mm as determined by the grip humidity or in liquid water are published in the open literature. separation. Intermediate humidity and the coupled effects of temperature To achieve the desired environmental conditions in the cham­ and humidity have yet to be studied. ber the temperature was increased to the set point and allowed to State-of-the-art PEMFCs, such as Nafion® membranes1 and stabilize, where after the humidity was slowly increased to the GORE-SELECT® membranes,2 utilize perfluorosulfonic acid desired RH-set point. During this process, the length of the spec­ polymers. In this paper, influences of temperature and humidity imen changes due to the thermal and swelling expansions of the on the mechanical properties of Nafion®112 are systematically membrane. Before testing, the crosshead was manually adjusted studied. The properties studied are the Young’s modulus, pro­ until the initial load applied to the specimen was brought back portional limit stress, break stress and the elongation at break. to zero, eliminating the slack caused by thermal and swelling The in-plane swelling behavior of the membrane at different expansions. Thus, the gauge length of the specimen was now temperatures and humidity is also investigated. the original length plus the displacement of the crosshead. The recorded value of the crosshead change is taken as a measure 2. Experimental of the dimensional change of the membrane due to a change in temperature and humidity. For all tests, the strain rate was 2.1. Preparation of membrane samples 0.2 mm/mm per minute. 3. Results and discussions The perfluorosulfonic acid membrane, Nafion®112 was used in all of the experiments. Nafion®112 has the following phys­ 3.1. Stress–strain curves at different temperature and ical properties: a nominal equivalent weight of 1100 g/mol of humidity sulfonic group and a thickness of two thousandths of an inch (52 �m). Prior to use, the membrane was pre-treated by boiling, Fig. 3 shows typical results for the engineering stress–strain for one hour each, in 3% hydrogen peroxide, 0.5 M sulfuric acid ® and deionized water (DI). Between each step, the membrane was curves of Nafion 112 along the transverse direction at the fixed relative humidity of 50% for a range of temperatures (the stress–strain curves along machine direction are similar to 1 Nafion® is a registered trademark of E.I. DuPont De Nemours & Co. transverse direction). As the temperature decreases, the curves 2 GORE-SELECT® is a registered trademark of W.L. Gore & Associates, Inc. monotonically shift upward corresponding to increasing tensile Fig. 2. (A) Experimental equipments: MTS alliance RT/5 material testing system with custom-designed environmental chamber from ESPEC Inc. (B) Tensile tests of the membrane. Specimen was aligned with the extension rod by a pair of vise-action grips. strength. The elongation at break increases with increasing cantly. Although the tensile strength decreases as the humidity temperature. Fig. 4 shows the stress–strain curves along the increases, it is not clear if the break stress and the elongation at transverse direction at fixed temperature 45 ◦C, for a range of break point are affected much by humidity change. Figs. 3 and 4 relative humidities. At higher humidity, more water is absorbed indicate that, in the range of values studied, temperature gener­ in the hydrophilic sulfonic acid clusters, which weaken the ally has a more significant effect on the mechanical behavior of ionic interactions and change the mechanical stability signifi- Nafion® membrane than humidity.

Fig. 3. Engineering stress–strain curves of tensile tests at different temperatures Fig. 4. Engineering stress–strain curves of tensile tests at different relative at 50% relative humidity (transverse direction). humidity at 45 ◦C (transverse direction). Fig. 5. Definition of proportional limit stress and Young’s modulus.

Based on these monotonic engineering stress–strain curves Fig. 6. Young’s modulus as a function of temperature at various relative humidi­ of tensile tests (Figs. 3 and 4), it is not possible to identify the ties: (A) transverse direction and (B) machine direction. onset of yielding. Instead, a “proportional limit” stress is defined in this paper to describe the onset of non-linearity. To determine Figs. 8 and 9, we see that the proportional limit stress of Nafion® the proportional limit stress, the tangents to the regions on either membrane decreases as the temperature and humidity increase. side of the “bend” are extended until they intersect, as indicated Although relative humidity apparently affects Young’s mod­ in Fig. 5. This point is defined as the proportional limit stress. ulus and the proportional stress, no obvious effects can be seen Young’s modulus is taken as the linear regression of the initial for break stress and break strain (Figs. 10 and 11). However, linear part of stress–strain curve. higher temperature does tend to cause to lower break stress and may lead to higher break strain, at least in the machine direction 3.2. Effects of temperature and humidity on the mechanical (Figs. 12 and 13). properties of the membrane Fracture surface morphologies of two extreme cases are shown in Figs. 14 and 15 (temperature at 25 ◦C and humidity at The Young’s modulus, proportional limit stress, break 30% (T25-RH30), and temperature at 85 ◦C and humidity at 90% stress and break strain are determined from each engineering stress–strain curve and the average value for each of these param­ eters is evaluated at each temperature–humidity combination. These average values are plotted in Figs. 6–13 to show each parameter as a function of either temperature or relative humid­ ity. Properties in both the transverse and the machine directions have been investigated. The effects of temperature and humidity on the Young’s mod­ ulus on both the machine and transverse directions are shown in Figs. 6 and 7, respectively. The results indicate that higher temperature and relative humidity lead to lower Young’s modu­ lus. Since water has a very low glass transition temperature Tg (estimated value at −130 ◦C) [11], it acts as a very good plas­ ticizer even in small quantities. In the presence of water, the interference between water and the chain-to-chain secondary bonding reduces the intermolecular forces. As a result, chains acquire greater mobility and the free volume increases, lead­ ing to a decrease in the glass transition temperature and the strength [13]. Although the glass transition temperatures of perflurosulfonic acid membranes such as Nafion® are usually between 100 and 150 ◦C [14], with increasing temperature, the amorphous domain (with higher strength) decreases, leading Fig. 7. Young’s modulus as a function of humidity at various temperatures: (A) to lower Young’s modulus and proportional limit stress. From transverse direction and (B) machine direction. Fig. 10. Break stress as a function of humidity at various temperatures: (A) Fig. 8. Proportional limit stress as a function temperature at various relative transverse direction and (B) machine direction. humidities: (A) transverse direction and (B) machine direction.

(T85-RH90)). Both show a typical ductile polymer tensile frac­ and break stress on the machine direction are all slightly larger ture surface [15], however, at higher magnification (Fig. 15), the than those on the transverse direction. The break strain in the fracture surface of the membrane tested at 25 ◦C and 30% RH machine direction, however, is smaller than that in the transverse shows smaller “cavities” and corresponding “pull-outs”, thus direction. indicating a less ductile behavior than for the membrane tested at 85 ◦C and 90%. 3.3. Swelling behaviors of the membrane at different The mechanical property changes with respect to temper­ temperatures ature and humidity, in the transverse direction are similar to those on the machine direction. However, at the same temper­ Dimensional changes with relative humidity at different tem­ ature and humidity, the Young’s modulus, proportional stress peratures along the transverse and machine directions are shown

Fig. 9. Proportional limit stress as a function of humidity at various tempera- Fig. 11. Break strain as a function humidity at various temperatures: (A) trans­ tures: (A) transverse direction and (B) machine direction. verse direction and (B) machine direction. Fig. 12. Break stress as a function temperature at various relative humidities: (A) transverse direction and (B) machine direction.

Fig. 14. SEM images at low magnification of the fracture surface of the mem­ brane at: (A) temperature 25 ◦C and relative humidity 30%; (B) temperature in Fig. 16. With increasing humidity, the swelling coefficient 85 ◦C and relative humidity 90%. (the derivative of the curve) increases; the swelling coefficient at higher relative humidity, above 70%, increases dramatically 3.4. Effect of pre-treatment procedures on the membrane compared with the intermediate humidity range. It is clear from properties Fig. 16 that the swelling coefficients at higher temperatures are larger than that at lower temperatures. At the same tempera­ Finally, we compare the Young’s modulus at 50% RH of ture and humidity, the dimensional change percentage along the the as-received membrane with that of the pre-treated mem- transverse direction is larger than that along the machine direc­ tion, although the trends are similar.

Fig. 15. SEM images at higher magnification of the fracture surface of the Fig. 13. Break stain as a function temperature at various relative humidities: (A) membrane at: (A) temperature 25 ◦C and relative humidity 30%; (B) temperature transverse direction and (B) machine direction. 85 ◦C and relative humidity 90%. treatment steps also promotes creation and growth of hydrated ionic clusters which leads to an increase in water content of the membrane. The rejuvenation of the membrane, along with the formation of ionic clusters, is responsible for the higher water content of pre-treated samples [16]. Comparing with as-received Nafion® membranes, pre-treated samples have lower tensile strength at the same temperature and humidity conditions.

3.5. Kruskal–Wallis tests of the experimental results

A nonparametric statistical analysis, Kruskal–Wallis test [17], has been applied to the experimental results in order to examine the significance of temperature and humidity effects on the mechanical properties of the tested material. The Kruskal–Wallis test is primarily a rank test where all the data for a given temperature or relative humidity is collected and ordered from the smallest to the largest value, regardless of the group which the data came from. An H-value can be found by applying Kruskal–Wallis equation: Fig. 16. Dimensional changes as a function of relative humidity at various tem­ peratures: (A) transverse direction and (B) machine direction. �k R2 H = 12 i − n + n n + n 3( 1) (1) ( 1) i= i brane in Fig. 17. It is evident that the as-received membrane 1 has higher Young’s modulus than that of the pre-treated mem­ In which ni (i =1, 2, ..., k) represent the sample sizes for each of brane. The pre-treatment procedure in this study involves boil­ the k groups (i.e.,� samples) in the data. Ri is the sum of the ranks ing membrane samples in hydrogen peroxide, sulfuric acid n = k n for group i. i=1 i. Each of the ni should be at least 5 for solutions, and deionized water, as described in Section 2. the approximation to be valid. This statistic approximates a χ2­ According to Zawodzinski et al. [16], whereas the conduc­ distribution with k − 1 degrees of freedom under the significance ® tivity of Nafion membrane is improved upon pre-treatment level alpha (typically set to 0.05). The corresponding χ2-value due to removal of low-molecular weight impurities left dur­ can be found in the χ2-distribution table [17]. In this study, the ® ing processing, boiling Nafion membrane during the pre- χ2-value is 7.815. If the H-value is greater than the χ2-value, the input being tested is significantly affecting the data. Table 1 lists the H-values of the humidity effects on the membrane properties in the transverse direction at four different temperatures. From the results, it is clear that Young’s modulus and proportional limit stress of Nafion® are affected by humidity significantly, however, the break stress and break strain of the membrane are not affected much. The H-values from Kruskal–Wallis tests on the temperature effects at four different humidities are listed in Table 2. These results show that temperature change affects the Young’s modulus, proportional limit stress and break stress. However, the break strain is not significantly affected under tem­ perature change. A similar analysis on the mechanical properties in the machine direction showed that the same conclusions are valid for this set of materials.

Table 1 The Kruskal–Wallis statistical H-values for humidity at various temperatures* Temperature Young’s Proportional Break stress Break strain (◦C) modulus limit stress

25 15.647 14.763 3.360 3.274 45 16.592 15.303 3.309 3.753 65 14.776 16.282 1.571 1.355 85 14.951 10.432 9.477 3.994 Fig. 17. Comparison of Young’s modulus at 50% relative humidity between the as-received membrane and the membrane which was pre-treated. * A value greater 7.815 indicates statistical significance for this sample set. Table 2 Acknowledgements The Kruskal–Wallis statistical H-values for temperature at various humidities* Relative Young’s Proportional Break stress Break strain This research has been supported by grants from the U.S. humidity (%) modulus limit stress Department of Energy, and W.L. Gore & Associates, Inc. The ® 30 15.109 17.108 13.703 9.852 Nafion 112 used in this study was generously donated by 50 16.895 16.592 15.071 8.136 DuPont. We thank Dr. Lidia Rejto of University of Delaware 70 17.857 17.857 14.314 5.811 for her advice on the statistical analysis of the experiments data. 90 13.150 12.829 8.725 0.079

* A value greater 7.815 indicates statistical significance for this sample set. References

4. Concluding remarks [1] J.B. Benziger, I.G. Kevrekidis, Proceedings of the AIChE Annual Meet­ ing, San Francisco, CA, 2003. [2] U.G. Bossel, Proceedings of the European Fuel Cell Forum Portable The mechanical properties of perfluorosulforic acid (PFSA) Fuel Cells Conference, Lucerne, 1999, pp. 79–84. membrane Nafion®112 have been investigated at different [3] Y. Lai, C.K. Mittelsteadt, C.S. Gittleman, D.A. Dillard, Proceedings of humidities and temperatures in a custom-designed environmen­ Third International Conference on Fuel Cell Science, Engineering and tal chamber. The results show that Young’s modulus and the Technology, Ypsilanti, Michigan, May 23–25, 2005. [4] Y. Tang, M.H. Santare, A.M. Karlsson, S. Cleghorn, W.B. Johnson, J. proportional limit stress of the membrane decrease as humid­ Fuel Cell Sci. Technol., in press. ity and temperature increase. Higher temperature leads to lower [5] A. Kusoglu, A.M. Karlsson, M.H. Santare, S. Cleghorn, W.B. Johnson, break stress and higher break strain. However, humidity has little submitted for publication. effect on the break stress and elongation at break. The dimen­ [6] V. Stanic, Proceedings of the Fourth International Symposium on Proton sional changes of the membrane at different temperature and Conducting Membrane Fuel Cells, October 2004. [7] W. Liu, K. Ruth, G. Rusch, J. New Mater. Electrochem. Syst. 4 (2001) humidity are also studied. Membranes at higher temperature 227–231. experience higher swelling coefficient. [8] A. Webber, J. Newman, AIChE J. 50 (12) (2004) 3215–3226. Only a minor, but consistent, difference in mechanical proper­ [9] Y. Tang, M.H. Santare, A.M. Karlsson, S. Cleghorn, W.B. Johnson, ties was detected between the transverse and machine directions: Proceeding of the Third International Conference on Fuel Cell Science, the Young’s modulus, proportional stress and break stress in Engineering and Technology, Ypsilanti, Michigan, May 23–25, 2005. [10] S. Cleghorn, J. Kolde, W. Liu, in: W. Vielstich, H.A. Gasteiger, A. the machine direction are all larger than those on the transverse Lamm (Eds.), Handbook of Fuel Cells—Fundamentals Technology and direction at the same temperature and humidity. The break strain Applications, John Wiley & Sons, Ltd., 2003. in the machine direction is smaller than that in the transverse [11] J.T. Uan-Zo-li, The effects of structure, humidity and aging on the direction. The dimensional change percentage along the trans­ mechanical properties of polymeric ionomers for fuel cell applications, verse direction is larger than that along the machine direction, Master Thesis, Virginia Tech., 2001. [12] R.C. McDonald, C.K. Mittelsteadt, E.L. Thompson, Fuel Cell 4 (3) although the trends are similar. (2004) 208–213. The statistical analyses show consistent temperature and [13] C.A. Daniels, Polymers: Structure and Properties, vol. 2, Technomic humidity effects on the membrane mechanical properties: Publishing Company, Inc., Lancaster, PA, 1989, pp. 21–22. the temperature and relative humidity significantly affect the [14] S.C. Yeo, A. Eisenberg, J. Appl. Polym. Sci. 21 (1977) 875–898. Young’s modulus and proportional limit stress. Additionally, the [15] K. Liu, M.R. Piggott, Polym. Eng. Sci. 38 (1) (1998). [16] T.A. Zawodzinski, C. Derouin, S. Radzinski, R.J. Sherman, V.T. Smith, break stress was significantly affected by the temperature but not T.E. Springer, S. Gottesfeld, J. Eletrochem. Soc. 140 (4) (1993) 1041. the humidity and the break strain was not significantly affected [17] R.E. Walpole, Probability and Statistics for Engineers and Scientist, by either the temperature or the relative humidity. Maxwell Macmillan International, 1993.